Optical fibers can be designed for single mode operation at specified wavelengths such as 1.3 .mu.m or 1.55 .mu.m. However, in order to be suitable for use in long distance, high bit rate transmission systems, a single mode fiber must also have low Rayleigh scattering losses, low bending-induced losses, and low chromatic dispersion. Recent experimental studies have indicated that a step-index fiber, with a germanium doped 7.5 .mu.m core diameter, a 110 .mu.m silica cladding diameter and an index difference .DELTA.=0.5 percent, provides very low loss and very good resistance to bending-induced (i.e., cabling) loss. However, minimum dispersion occurs near 1.35 .mu.m, which is too far removed from the preferred operating wavelength of 1.3 .mu.m. In this regard germania-fluoro-phosphosilicate depressed-index cladding lightguides are attractive alternative structures because the germania dopant used to increase the core index serves to increase the zero material dispersion wavelength, whereas the fluorine used to decrease the cladding index has the effect of decreasing the material dispersion at the longer wavelengths such as 1.55 .mu.m. As a result, the contributions to chromatic dispersion by the two dopant materials can be counterbalanced, and the total chromatic dispersion can be minimized at any wavelength within the desired range between 1.28 and 1.38 .mu.m, for core diameters between 6 .mu.m and 10 .mu.m. Therefore, fiber waveguide parameters, such as core diameter and index difference, can be chosen to minimize losses, whereas dispersion effects can be minimized by choosing the proper dopant concentrations.
It has been observed, however, that depressed-index-cladding lightguides have high losses at the longer wavelengths. Thus, a lightguide designed to operate at 1.3 .mu.m could not be useful at 1.55 .mu.m. As a result, the use of a 1.3 .mu.m fiber in any system would limit the ability of the system to grow by simultaneous operation at both 1.3 .mu.m and at some longer wavelength such as 1.55 .mu.m.